The present invention relates to the field of communication devices that communicate using radiation of electromagnetic energy through antennas and particularly relates to portable phones, pagers and other telephonic devices.
Personal communication devices, when in use, are usually located close to an ear or other part of the human body. Accordingly, use of personal communication devices subjects the human body to radiation. The radiation absorption from a personal communication device is measured by the rate of energy absorbed per unit body mass and this measure is known as the specific absorption rate (SAR). Antennas for personal communication devices are designed to have low peak SAR values so as to avoid absorption of unacceptable levels of energy, and the resultant localized heating by the body.
For personal communication devices, the human body is located in the near-field of an antenna where much of the electromagnetic energy is reactive and electrostatic rather than radiated. Consequently, it is believed that the dominant cause of high SAR for personal communication devices is from reactance and electric field energy of the near field. Accordingly, the reactance and electrostatic fields of personal communication devices need to be controlled to minimize SAR.
Antennas Generally
In personal communication devices and other electronic devices, antennas are elements having the primary function of transferring energy to or from the electronic device through radiation. Energy is transferred from the electronic device into space or is received from space into the electronic device. A transmitting antenna is a structure that forms a transition between guided waves contained within the electronic device and space waves traveling in space external to the electronic device. A receiving antenna is a structure that forms a transition between space waves traveling external to the electronic device and guided waves contained within the electronic device. Often the same antenna operates both to receive and transmit radiation energy.
J. D. Kraus xe2x80x9cElectromagneticsxe2x80x9d, 4th ed., McGraw-Hill, New York 1991, Chapter 15 Antennas and Radiation indicates that antennas are designed to radiate (or receive) energy. Antennas act as the transition between space and circuitry. They convert photons to electrons or vice versa. Regardless of antenna type, all involve the same basic principal that radiation is produced by accelerated (or decelerated) charge. The basic equation of radiation may be expressed as follows:
IL=Qxcexd(Am/s)
where:
I=time changing current (A/s)
L=length of current element (m)
Q=charge (C)
xcexd=time-change of velocity which equals the acceleration of the charge (m/s)
The radiation is perpendicular to the direction of acceleration and the radiated power is proportional to the square of IL or Qxcexd.
A radiated wave from or to an antenna is distributed in space in many spatial directions. The time it takes for the spatial wave to travel over a distance r into space between an antenna point, Pa, at the antenna and a space point, Ps, at a distance r from the antenna point is r/c seconds where r=distance (meters) and c=free space velocity of light (=3xc3x97108 meters/sec). The quantity r/c is the propagation time for the radiation wave between the antenna point Pa and the space point Ps.
An analysis of the radiation at a point Ps at a time t, at a distance r caused by an electrical current I in any infinitesimally short segment at point Pa of an antenna is a function of the electrical current that occurred at an earlier time [txe2x88x92r/c] in that short antenna segment. The time [txe2x88x92r/c] is a retardation time that accounts for the time it takes to propagate a wave from the antenna point Pa at the antenna segment over the distance r to the space point Ps.
Antennas are typically analyzed as a connection of infinitesimally short radiating antenna segments and the accumulated effect of radiation from the antenna as a whole is analyzed by accumulating the radiation effects of each antenna segment. The radiation at different distances from each antenna segment, such as at any space point Ps, is determined by accumulating the effects from each antenna segment of the antenna at the space point Ps. The analysis at each space point Ps is mathematically complex because the parameters for each segment of the antenna may be different. For example, among other parameters, the frequency phase of the electrical current in each antenna segment and distance from each antenna segment to the space point Ps can be different.
A resonant frequency, ƒ, of an antenna can have many different values as a function, for example, of dielectric constant of material surrounding antenna, the type of antenna and the speed of light.
In general, wave-length, xcex, is given by xcex=c/ƒ=cT where c=velocity of light (=3xc3x97108 meters/sec), ƒ=frequency (cycles/sec), T=1/ƒ=period (sec). Typically, the antenna dimensions such as antenna length, At, relate to the radiation wavelength xcex of the antenna.
The electrical impedance properties of an antenna are allocated between a radiation resistance, Rr, and an ohmic resistance, Ro. The higher the ratio of the radiation resistance, Rr, to the ohmic resistance, Ro the greater the radiation efficiency of the antenna.
Antennas are frequently analyzed with respect to the near field and the far field where the far field is at locations of space points Ps where the amplitude relationships of the fields approach a fixed relationship and the relative angular distribution of the field becomes independent of the distance from the antenna.
Antenna Types
A number of different antenna types are well known and include, for example, loop antennas, small loop antennas, dipole antennas, stub antennas, conical antennas, helical antennas and spiral antennas. Such antenna types have often been based on simple geometric shapes. For example, antenna designs have been based on lines, planes, circles, triangles, squares, ellipses, rectangles, hemispheres and paraboloids. Small antennas, including loop antennas, often have the property that radiation resistance, Rr, of the antenna decreases sharply when the antenna length is shortened. Small loops and short dipoles typically exhibit radiation patterns of xc2xdxcex and xc2xcxcex, respectively. Ohmic losses due to the ohmic resistance, Ro are minimized using impedance matching networks. Although impedance matched small loop antennas can exhibit 50% to 85% efficiencies, their bandwidths have been narrow, with very high Q, for example, Q greater than 50. Q is often defined as (transmitted or received frequency)/(3 dB bandwidth).
An antenna goes into resonance where the impedance of the antenna is purely resistive and the reactive component is 0. Impedance is a complex number consisting of real resistance and imaginary reactance components. A matching network forces a resonance by eliminating the reactive component of impedance for a particular frequency.
Antennas based upon more complex shapes have also been proposed. For example, U.S. Pat. No. 6,104,349 to Cohen and entitled TUNING FRACTAL ANTENNAS AND FRACTAL RESONATORS describes dipole antennas based upon deterministic fractals. Fractals are patterns based upon a plurality of connected segments. Fractal patterns are categorized as random fractals (which are also termed chaotic or Brownian fractals) or deterministic fractals. A deterministic fractal is a self-similar structure that results from the repetition of a design (sometimes called a xe2x80x9cmotifxe2x80x9d or xe2x80x9cgeneratorxe2x80x9d).
Low SAR Antennas
Antenna design involves tradeoffs between antenna parameters including gain, size, efficiency, bandwidth and SAR. When antennas are employed in personal communication devices, size is of paramount importance since the antenna must not be physically obtrusive to the user and SAR must be low to minimize local heating in the body of users.
U.S. Pat. No. 5,784,032 to Johnston et al entitled COMPACT DIVERSITY ANTENNA WITH WEAK BACK NEAR FIELD described three-dimensional antennas with multiple diversity interconnected loops that are described as having weak near fields. However, three-dimensional antennas are somewhat difficult to design into the physical enclosure of compact personal communication devices while still obtaining acceptable parameter values.
In consideration of the above background, there is a need for improved antenna designs that achieve the objectives of low values of SAR, physical compactness suitable for personal communication devices and other acceptable antenna design parameters.
The present invention is a segmented loop antenna formed of many segments connected in an electrical loop where the segments are arrayed in multiple divergent directions that tend to increase the antenna electrical length while permitting the overall outside antenna dimensions to fit within the antenna areas of communication devices.
The loop antenna operates in a communication device to exchange energy at a radiation frequency and includes a connection having first and second conductors for conduction of electrical current in a radiation loop. The radiation loop includes a plurality of electrically conducting segments each having a segment length. The segments are connected in series electrically connected between said first and second conductors for exchange of energy at the radiation frequency. The loop has an electrical length, At that is proportional to the sum of segment lengths for each of said radiation segments and the segments are arrayed in a pattern so that different segments connect at vertices and conduct electrical current in different directions near the vertices.
The arrayed segments that form the loop antenna may be straight or curved and of any lengths. Collectively the arrayed segments appreciable increase antenna electrical lengths while permitting the antenna to fit within the available area of communicating devices. The pattern formed by the antenna segments may be regular and repeating or may be irregular and non-repeating. Mathematically, the pattern of the arrayed-segment loop antenna may be expressed as a continuous function or as a discontinuous function with one or more, and frequently many, directional discontinuities that collectively increase the antenna electrical length while maintaining overall external dimensions of the loop antenna.
The electrical length of the arrayed-segment loop antenna is typically equal to the wavelength, xcex, or integral multiples thereof, of the radiation wave from the antenna. Although the antenna""s electrical length is not small compared to xcex, the near field in reactive and electrical fields tend to be low whereby the SAR for the arrayed-segment loop antenna tends to be low.
The arrayed-segment loop antennas are typically located internal to the housings of personal communicating devices where they tend to be less immune to de-tuning due to objects in the near field in close proximity to the personal communicating devices.
The foregoing and other objects, features and advantages of the invention will be apparent from the following detailed description in conjunction with the drawings.